My neighbor across th street had a retaining wall installed over the summer. Yesterday we had a ton of rain on top ofvery wet conditions. About a 20 foot section fell over. Inspecting it i noticed there was no pea gravel behind it. No drainage at the bottom to allow water to escape. Now I am no expert as I have never installed a retaining wall, but don't you have to have these two items behind the wall to prevent just what happened? by the way the wall is about 5 foot high all the way across.